The present invention relates to the extraction or recovery of metal values from feed materials and more particularly to the extraction or recovery of metal values from feeds not amenable to efficient extraction by conventional methods.
It is well known that many non-ferrous metal sulphide materials may be dissolved in nitric acid or other acidic solutions containing an oxidizing agent comprising dissolved oxides of nitrogen. A nitric acid-sulphuric acid mixture is one such acidic solution. Examples of such prior art include European Patent Application 87310905.2 (Electrolytic Zinc Company of Australasia Limited). This is typical of the known prior art in that it relates to the oxidative leaching of metal sulphides such as zinc sulphide concentrates.
The known prior art is typically concerned with the treatment of copper- and zinc-containing sulphides. These feeds can be classed as non-refractory being relatively simple feeds to process. However, many feeds particularly those that are the intermediate products of other processes including hydrometallurgical and pyrometallurgical processes are generally considered difficult to treat. With such difficult-to-treat feeds the dissolution of the valuable components may be inhibited, for example, by passivation. Accordingly they are not amenable to extraction or recovery by conventional methods such as direct treatment with aqueous solutions containing nitric acid or sulphuric acid.
Therefore, it has hitherto been considered that in order to achieve acceptable dissolution rates with such feeds, some form of pre-treatment such as roasting is required. Such pre-treatment is costly in terms of both capital costs and operating costs, may cause the dissolution of undesirable components of the feed and may not yield complete dissolution of the desired metals. Alternatively, processing conditions may need to be extremely aggressive to achieve commercially viable rates of reaction.
Processing difficulties are compounded when the feed contains arsenic. Safe disposal of arsenic-containing residues is an important consideration. Many countries have strict mandatory limits on the allowable arsenic levels of wastes from processing operations which are intended to be held in tailings dams or otherwise disposed of.
One conventional method of processing arsenic-containing feeds is by ferric sulphate leaching. During processing of arsenic-containing feeds by ferric sulphate leaching, only a portion of the arsenic (III) is generally oxidized to arsenic (V). The arsenic (III) remains soluble in the aqueous processing media while the arsenic (V) is precipitated as a hydrated ferric arsenate (ideally Fe AsO.sub.4.xH.sub.2 O). It is known to use an excess of soluble iron in the arsenic precipitation circuit to inhibit the redissolving of arsenic from the ferric arsenate precipitate which is typically disposed of in tailings dams. Typically the Fe/As molar ratio in the precipitate must be at least 4:1. The soluble arsenic (III) which is present in the leach liquor needs to be removed before the metal values can be recovered or excess process water disposed of in an environmentally acceptable manner. Such additional process steps involve the use of additional reagents and can lead to significant increases in both capital and operating costs.
Precipitation of an environmentally stable arsenic-containing product is difficult to achieve with feeds in which the iron content is below that required for the precipitation of an iron arsenate with a Fe/As ratio of 4/1 or above. Previous practice has been to provide sufficient calcium cations to precipitate calcium arsenate for "total" arsenic precipitation. At the same time the pH of the slurry is adjusted to the appropriate range. However, it is known that calcium arsenate is more chemically reactive than ferric arsenate and is not regarded as an acceptable alternative.
More commonly, and as well established by prior art, non-ferrous metal sulphides can be dissolved in acidic sulphate solutions. Such solutions can be formed directly from sulphuric acid, by bacterial regeneration of ferric sulphate solutions, or more commonly by the oxygen pressure leaching technology.
U.S. Pat. Nos. 4,244,732 (Reynolds), 4,244,735 (Reynolds) and 4,331,469 (Kunda), all provide for disposal of arsenic in feed materials as an iron arsenic compound. However, these processes require pressure oxidation to operate in the sulphuric acid environment provided. These prior proposals are accordingly not cost effective and/or do not yield environmentally acceptable arsenic-containing by-products.